(a) Field of the Invention
This invention relates to an optical information recording disk adapted for use in reading out signals, which have been recorded therein, by means of a laser beam to be reflected by or transmitted through the disk.
(b) Description of the Prior Art
An optical information recording/reproducing system has been proposed and adopted to a certain extent. In accordance with the above system, signals, which have been recorded on a disk by means of minute pits formed therein, are read out by directing a spot beam of a laser beam onto the disk and detecting the intensity of a beam reflected by or transmitted through the disk. A widespread use of the above system for the recording/reproducing of picture images, voice and sound as well as the storage/reproduction of a great number of pieces of information is expected, since it permits to increase the packing density considerably, the quality of picture images, voice or sound to be reproduced therefrom are excellent in their characteristics, and a great number of copies can be easily produced, for example, using a single stamper.
Needless to say, the recording of information, which makes use of the above principle, is not necessarily limited to such disk-like configurations but may also be effected by means of a plane or curved surface of any desired configuration if necessary; and such are included by the disk of the present invention.
The base material of the disks for use in the above recording/reproducing system are required, of course, to be transparent so as to allow a laser beam to transmit through. They are also required, strongly, to have a high degree of optical uniformity. It has been well-known that, depending on the resin to be used as a base material for disks, the resin may undergo a flow upon molding and molecular orientation in the course of its cooling, whereby causing birefringence to occur. The optical non-uniformity resulting from birefrigence is a fatal defect for optical disks. From such a viewpoint, the base material of optical disks has been limited to certain specific resins. In addition, it is essential that such a base material of optical disks have good moldability so as to replicate minute pits of a stamper. As resinous materials capable of satisfying both the high degree of optical uniformity mentioned above and good moldability, only polymers containing methyl methacrylate as their principal components from a practical standpoint, shall be referred to.
However, optical disks made of conventionally-known polymers or copolymers of methyl methacrylate (their principal components) are accompanied by a fatal drawback that they are susceptible to warping or twisting especially when placed in an environment of high humidity. Improvement of such a drawback has thus been eagerly awaited.